Setting a particular address for a monitor causes the system to static route it through a particular gateway. Setting one of the system DNS servers to a particular gateway also causes the system to static route it. If you are not careful, you can cause the monitor to flap between two gateways. From your description, it sounds possible you have encountered this problem.

I know OPNsense share very little with pfsense but the logic may be the same.

You will get errors in the log because the system will set a route for DNS (separate script) and also for monitoring (separate script). I'd not do this! There are more public dns servers that 8.8.8.8 and 8.8.4.4

No cable has been moved.Right now everything is running with the same cable setting but with OPNsense instead of fpSense.Anyway, this isn't a OPNsense related issue.I'll report back if the same issue will show-up with OPNsense.Thank you for the support.